The brightness and whiteness of printable paper may be improved, for example, by treating the surface of the paper substrate with optical brightening agents (OBAs). OBAs are fluorescent materials which increase the brightness (e.g., white appearance) of paper substrate surfaces by absorbing the invisible portion of the light spectrum (e.g., from about 340 to about 370 nm) and converting this energy into the longer-wavelength visible portion of the light spectrum (e.g., from about 420 to about 470 nm). In other words, the OBAs convert invisible ultraviolet light and re-emits that converted light in the blue to blue-violet light region through fluorescence.
In improving the brightness or whiteness of paper substrates, OBAs often operate by compensating for a yellow tint or cast which may be present in paper substrates prepared from paper pulps which have, for example, been bleached to moderate levels. This yellow tint or cast is caused by the absorption of short-wavelength light (violet-to-blue) by the paper substrate. By treating the paper substrates with OBAs, this short-wavelength light causing the yellow tint or cast may be partially replaced, thus improving the brightness and whiteness of the paper substrate.
The use of OBAs in improving the brightness or whiteness of paper substrates is not without problems. These OBAs may interact or react with other chemicals used in papermaking. For example, many OBAs used in papermaking are anionic. Illustrative of these anionic OBAs are the stilbene-based sulfonates. By contrast, some of the chemicals used in papermaking are cationic, or have cationic moieties. These cationic chemicals used in papermaking may interact or react with these anionic OBAs such as the stilbene-based sulfonates. Such interactions or reactions may reduce the ability of these OBAs to optically brighten and whiten the paper substrate.